In the manufacture of a metal oxide semiconductor (MOS) device, an insulation, i.e., a dielectric layer, is formed on a surface of the semiconductor substrate. This insulation layer may be silicon dioxide or silicon nitride, which is typically used as a gate dielectric for the MOS device. For purposes of quality control, the thickness of the insulation layer must be within a specified range. Unfortunately, once the insulation layer is exposed to an ambient environment, a contamination layer is formed on the insulation layer. This contamination layer grows over time to a predicted thickness based upon particles in the ambient environment, as well as the amount of humidity present in the ambient environment.
Measurement tools, such as an ellipsometer, can not distinguish between the insulation layer and the contamination layer because they have similar optical properties. Consequently, an actual thickness of the insulation layer may be within a specified range, yet the measured thickness exceeds this range because of the added thickness of the contamination layer. One approach to this problem is to measure the insulation layer before being exposed to the ambient environment causing the contamination layer to form thereon. However, this is not always practical in manufacturing environments due to limited availability of the measurement tools.
Another approach is to expose or age the insulation layer in the ambient environment so that the contamination layer forms to a predicted level. However, the thickness of the contamination layer may still adversely effect the overall measured thickness of the insulation layer, particularly if the thickness of the contamination layer causes the apparent thickness of the insulation layer to exceed the specified range.
Yet another problem arises in the benchmarking of different measurement tools due to the contamination layer. After the insulation layer has aged, the thickness of the insulation layer is measured using a first measurement tool at a first manufacturing site. Afterwords, the semiconductor substrate may be shipped to a second manufacturing site for further processing. The location of the first and second manufacturing sites may be at respective widely spaced geographic locations. However, a second measurement tool used to measure the same thickness of the insulation layer at the second manufacturing site may not provide the same measured thickness as did the first measurement tool.
This discrepancy in the measured thickness of the insulation layer using the second measurement tool may be caused by a change in the thickness of the contamination layer. That is, if the amount of particles in the ambient environment changes between different manufacturing sites, this is reflected by the thickness of the contamination layer either increasing or decreasing. In other words, the contamination layer may vary over time. In addition, this discrepancy may be caused by the second measurement tool being out of adjustment.
Therefore, there is a need to measure the actual thickness of the insulation layer and not a combined thickness of the insulation and contamination layers so that thin film measurement tools can be benchmarked.